News, views, and reviews

Journal of Drugs in Dermatology, June, 2008 by Vitaly Terushkin, Isaac Brownell

News, Views, and Reviews provides focused updates, topic reviews, and editorials concerning the latest developments in dermatologic therapy.

Management of Methicillmrresistant Staphylococcus aureus (MRSA) Skin Infections and MRSA Colonization

Introduction

A substantial rise in community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) infections has led to major efforts to further characterize this growing problem. Much debate has recently centered on the management of patients with recurrent MRSA infections and those colonized with MRSA. The epidemiology, presentation, and treatment of MRSA skin infections are reviewed, and management approaches to asymptomatic MRSA carriers are discussed.

Background

Staphylococcus aureus (S aureus) isolates resistant to beta-lactam antibiotics are referred to as methicillin-resistant Staphylococcus aureus (MRSA). In 1961, 2 years after the introduction of methicillin, the first MRSA strains were discovered in British hospitals, and referred to as healthcare-associated MRSA (HA-MRSA). There was a steady increase in hospital infections with these organisms for the next 30 years, and it was not until the 1990s that MRSA infections began to originate from the community. A new term was coined for these isolates: community-associated MRSA (CA-MRSA). Community-associated MRSA is genetically distinct from HA-MRSA. Specifically, CA-MRSA carries a unique type of gene complex known as staphylococcal cassette chromosome mec (SCCmec), containing the mecA methicillin resistance gene. In addition, CA-MRSA contains genes encoding the Panton-Valentine leukocidin, an exotoxin that increases the risk of abscess formation and necrosis of the skin, and thus accounts for its predilection toward skin and soft tissue infections (SSTIs). Although many genotypes have been found in the CA-MRSA group, USA300 is the most common CA-MRSA strain in the US. (1)

Individuals prone to CA-MRSA infections are usually found in close-contact settings, such as households, day-care centers, military centers, homeless shelters, and prisons. In addition, athletes, men who have sex with other men, intravenous drug users, Native Americans, and Pacific Islanders tend to have higher incidences of CA-MRSA. (1)

Cases of CA-MRSA infections are both widespread and common. One study assessed 422 patients presenting to emergency rooms with SSTIs in 11 different US cities, including Albuquerque, NM; Atlanta, Ga; Charlotte, NC; Kansas City, Kan; Los Angeles, Calif; Minneapolis, Minn; New Orleans, La; New York, NY; Philadelphia, Pa; Phoenix, Ariz; and Portland, Ore. Of 422 patients with skin infections, 320 cultures grew S aureus. The overall MRSA prevalence was 59% (range: 15%-74%). Most (98%) of the MRSA isolates carried the SCCmec type IV and the Panton-Valentine leukocidin gene, suggesting a community origin of the isolates. (2) Infections originating in rural settings have also been noted. In the 1990s, Native Americans living in rural Wisconsin experienced a number of MRSA outbreaks. In addition, there was an outbreak of CA-MRSA in southwestern Alaska in 1999. (3)

There has been a remarkable upswing in the incidence of CA-MRSA infections in recent years. At the Driscoll Children's Hospital in Corpus Christi, Tex, there was a 50-fold increase (from 9 to 459) in the number of CA-MRSA infections between 1999 and 2003.4 Similarly, 46 of 122 (38%) MRSA infections in children evaluated at a Memphis children's hospital were due to CA-MRSA rather than HA-MRSA from January 2000 to June 2001, with the proportion increasing to 106 of 167 (63%) during the subsequent 12 months. (5)

Skin and soft tissue infections, such as furuncles, abscesses, impetigo, and cellulites, are the most common CA-MRSA presentations. Community-associated MRSA may also present as a necrotic ulcer resembling a brown recluse spider bite. Other more severe presentations of CA-MRSA include necrotizing lung infections, necrotizing fascitis, toxic shock syndrome, and sepsis. (1)

The diagnosis of a CA-MRSA infection rests on specimens obtained for culture. A specimen can easily be recovered if the lesion is draining purulent matter. In the cases of a nonpurulent cellulitis, a biopsy with culture can be performed. Along with the culture results, antibiotic susceptibility tests should be done to further guide therapy.

Treatment of MRSA Infections

Skin and soft tissue infections are typically approached with 1 of 2 treatment strategies, the choiceof ( which depends on infection severity and associated systemic signs (ie, fever, hypotension, tachycardia). In patients with focal, uncomplicated SSTIs such as furuncles or small abscesses without systemic signs, incision and drainage (I&D) is often sufficient. In a retrospective cohort study I&D was successful in 87% (190 of 219) uncomplicated CA-MRSA SSTIs.6 However, when a patient presents with systemic signs or a larger purulent abscess, empiric antimicrobial therapy should be administered along with I&D. The decision to admit a patient for intravenous (IV) antibiotics depends on whether the patient fits 1 or more of the following criteria: presence of a large abscess, systemic signs, age younger than 6 months, diabetesor immunodeficiency. (1)